In the electro-mechanical transducer such as a speaker, a headphone, a microphone, or a hydrophone, a piezoelectric material has been utilized as a vibration film in view of the moldability, workability and mechanical properties thereof. For example, Japanese Patent Kokai No. 3734/73 discloses an electro-acoustic transducer in which a natural or synthetic polymer piezoelectric material is used as a vibrator element. The transducer comprises two thin film-like members made of polymer piezoelectric material and having entirely or partly piezoelectricity, at least two opposite ends of the members in longitudinal direction being fixed to support members having high rigidity. Between the piezoelectric members an auxiliary member is inserted for giving a deflecting stress to each piezoelectric member, and electrodes are provided on both side surfaces of the piezoelectric members. In Japanese Patent Kokai No. 56128/73 there has been proposed a piezoelectric speaker which includes a plate of the material having at least partly piezoelectric properties, an AC electric field being applied to the plate in the thickness direction and being converted into a mechanical vibration by the piezoelectric properties of the material, and the entire unit being operated as a sound radiator by the mechanical vibration.
Further, U.S. Pat. No. 4,088,915 discloses a piezoelectric electric-acoustic transducer using a diaphragm made of a piezoelectric film in which the diaphragm is supported by a support member having a curved portion to impact a suitable resiliency and/or tension to the diaphragm, thereby improving acoustic characteristics without reducing efficiency in the vibration of the diaphragm.
In general, it is known that when a large k.sub.33 (g.sub.33, d.sub.33) mode of an electro-mechanical conversion rate is utilized in a sheet-like piezoelectric element, the piezoelectric element is polarized in its lengthwise direction. In order to facilitate the polarization, a specific electrode arrangement may be used as will be described hereinafter.
In the prior art as mentioned above, since polarized voltages are applied to the surface portion of piezoelectric body or substrate, but no voltage is applied to the back portion thereof, irregular polarization occurs in the thickness direction of the piezoelectric substrate, with the result there is a drawback that the converting efficiency may be cause to deteriorate.
On the other hand, it is preferably to make the thickness of the piezoelectric body as thin as possible so as to uniformly apply the polarizing voltage in the thickness direction, but this has the disadvantage of reduced mechanical strength.
According to a recent development, there has been provided a polymer composite piezoelectric material having a piezoelectricity which is considerably higher than that of the conventional polymer piezoelectric material. Such composite piezoelectric material may usually be produced by dispersing piezoelectric ceramic powder having large self-polarization into a basic polymer, and has mechanical properties, moldability and workability which are similar to those of the basic polymer. Thus, the composite piezoelectric material can be easily formed in a film shape, with flexibility different from the conventional piezoelectric material.
It is, therefore, an object of the present invention to provide a sheet-like piezoelectric element which has an increased conversion efficiency and can be easily formed in an arbitrary shape.
Another object of the invention is to provide an electro-mechanical transducer which comprises said sheet-like piezoelectric element.
A further object of the invention is to provide a thermal-electric transducer which comprises said sheet-like piezoelectric element.